It has conventionally been known that silver, copper, zinc, and ions of these metals have an antibacterial activity. Typical application examples for metallic antibacterial agents utilizing the antibacterial activity of ions of these metals include a catheter having silver particles adhered to the surface thereof (see U.S. Pat. No. 4,054,139) and a method of coating a medical polymer material with a salt of a metal such as silver, zinc, or cerium (see U.S. Pat. No. 4,612,337 and JP-A-62-11457 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")). However, these prior art techniques have not been put to practical use, most likely due to poor particle properties, e.g., insufficient fineness or insufficient dispersibility, of the metal or other powder used.
Various antibacterial agents have been proposed which comprise inorganic compounds having ions of silver, copper or zinc adhered thereto or held thereon through ion exchange. Attempts are being made to incorporate ions of a metal such as silver into natural and synthetic zeolites through ion exchange and to use these zeolites as antibacterial agents in applications such as industrial articles, daily necessaries, or medical articles (see, for example, JP-B-63-54013 (the term "JP-B" as used herein means an "examined Japanese patent publication")).
In particular, an example of the synthesis of an antibacterial titania has been disclosed which comprises adding ammonia water to an aqueous solution of titanium trichloride in the presence of an alcohol to synthesize a white porous titania gel, and then causing this gel to adsorb antibacterial metal ions such as silver ions or/and zinc ions (see JP-A-5-4816). This antibacterial titania having antibacterial metal ions adsorbed thereto has an advantage in that since the antibacterial titania itself is dispersible into a colloidal state, an antibacterial composition containing the titania evenly and stably dispersed therein can be obtained.
Examples of use of antibacterial agents have been disclosed which include: a technique of dispersing such an antibacterial composition into a molten resin to form a resin molding having antibacterial properties; a technique of preparing a paint containing an antibacterial agent stably dispersed therein and applying the paint to a building to impart antibacterial properties thereto; a technique of adhering an antibacterial titania to fibers and using the fibers as a building material such as a wall paper; and a technique of incorporating an antibacterial titania into a resin and forming the resin into an antibacterial wrapping material for foods (see JP-A-4-231063). Further, an antibacterial titania (anatase titania having an antibacterial metal adhered thereto) having satisfactory antibacterial-metal dispersibility and sufficient particle fineness of the antibacterial agent itself and capable of being stably produced at low cost was disclosed recently (see JP-A-6-298532).
However, the antibacterial titania having adhered thereto silver ions, which are thought to have a relatively high antibacterial activity among ions of the metals enumerated above, has a drawback that since silver ions are unstable to ultraviolet rays and heat and are hence apt to be reduced into silver upon ultraviolet irradiation or heating, the color of the antibacterial titania containing silver ions changes into light black or gray. Consequently, the products containing an antibacterial agent comprising silver ions suffer discoloration upon exposure to light or heat resulting in an impaired commercial value, and hence they should be used in limited situations. Another drawback thereof is that the reduction of the silver ions adhered to titania into silver metal results in reduced antibacterial properties.